Despite the decades of experience with the design of chargers for lead acid batteries, a number of practical limitations on system performance have not been effectively or economically addressed. These limitations relate to specific parameters associated with cells, batteries, and/or battery assemblies (a group of serially connected batteries). The following discussion focuses on these specific parameters and how they effect the cells, batteries, and battery assemblies.
The problems with cells and batteries are essentially the same since a battery is a group of combined cells and are referred to as cells and/or batteries herein. These problems concern charging rate limits (the speed with which a cell can be charged without causing damage to the cell structure), float life limits (the length of time a cell or battery can be maintained on charge while inactive) and cyclic effects (the number of times a cell can be discharged and recharged to a suitable capacity). Battery system problems include the effects of differential capacity (different cells and/or batteries in a series assembly having different capacity) and differential evolution (individual cells and/or batteries in a series assembly aging at different rates) in cells and batteries which are members of a series "string".
With problems relating to both cell and battery systems, a major difference in emphasis results from the application in which they are employed. For example, standby power systems (such as telecommunications back up power systems) differ significantly from cyclic power systems (such as power systems for traction devices which undergo a deep cycle (routinely discharged to a low level) or load leveling (frequently discharged to a moderate depth of discharge). The construction of cells differs in many ways, depending on the type of service for which the cell is intended.
The following discussion begins with the problems relating to cells, progresses to batteries, and then to battery systems. The examples are provided to illustrate preferred methods of achieving improved performance at acceptable cost. Obviously, the cost/performance trade-offs will vary with the application and the risk associated with unsatisfactory battery performance.
An individual lead acid cell may be flat plate, rolled plate, thin or thick, jar or tank formed, starved gelled or flooded, sealed or unsealed and may have a variety of alloying agents, electrolyte additives and construction variants, i.e., separators, expanders, float energy or power density, cyclic or float life, deep or shallow service, economy or durability and so on.
The basic system still requires the passage of a net charging current in the reverse direction from discharge to recharge the cell. This requires both an excess voltage (over the equilibrium open circuit voltage) to achieve significant cell current, especially at high state-of-charge, and excess charge (current times time) to compensate for the less than 100% charge acceptance efficiency of the cell. Many parameters of cell construction, including plate alloying additives, active material porosity, electrolytic composition, specific gravity, volume and mobility, affect overvoltage requirements, voltage, charge efficiency, and the effect of charge rate (at a given state-of-charge) on cell life and recoverable capacity.
Generally, the ability of a cell to absorb charging current efficiently, without requiring a large voltage excess and with high charge storage efficiency, declines as the state-of-charge increases. A "C" rating is defined as the current rating in amps that will discharge a battery in one hour). A 1C charge rate into a battery that is at 10% of its full state-of-charge is harmless for virtually any cell construction. However, the same charge rate into a battery having a 90% state-of-charge necessitates substantial overvoltage and results in reduced charge storage efficiency, generation of significant cell heating, possibly local gassing and, in many cell structures, significant non-uniformities over the plate surfaces. These non-uniformities can cause substantial local morphological variations especially in the positive plate oxide. In addition, stratification effects can occur in tall vertical plates (above about 10 inches in height).
One means of partially overcoming certain of the previous mentioned deficiencies resulting from charging batteries having a high state-of-charge is the use of reverse (discharge) current pulses interspersed in the charging current. Reverse current pulses are categorized as long, short and very short in duration. They tend to enhance charge acceptance, especially at high states-of-charge, by lowering the "barrier" height (the amount of excess voltage needed for charge acceptance) and increasing the net recharge energy by reducing cell heating from moderate rate, late stage, charging conditions. The reverse current pulses have been used as an aid in "quick charging" lead acid batteries for decades, usually described as boundary layer depolarization. However, the use of reverse current pulses, both long and short, fall short of overcoming the limitations in battery charging discussed before.
The use of very short (in duration) reverse current pulses in the range of milliseconds has a modest effect on lowering barrier height and relatively little effect on charge acceptance. This is because the improvement in charge acceptance during current flow in the "forward" or charging direction must exceed the "reverse" or discharge "loss" of the "backward" pulse.
For short pulses in the range of seconds (but still very short compared to the cell step function equilibrium time, which itself depends on the state-of-charge and cell construction), little internal reactant redistribution is created, especially when the reverse pulse currents are comparable in magnitude to the charging current. However, small scale anomalies in conductivity, concentration, etc., tend to spontaneously disappear over a period of time under these conditions. For example, a small conductivity plate region which selectively concentrates charging current and thereby enhances local oxide evolution, would be likely to provide an enhanced current density during discharge as well as atypical efficiency in the reverse direction.
Obviously, the net charge and discharge can't balance if the battery is to be recharged, and short reverse current pulses will cancel a portion of the charging current. The macroscopic cancellation does not operate symmetrically at small dimensions, as evidenced by the net increase in charging voltage, despite the current "loss" from the reverse pulses. The net effect of short, moderate current, reverse pulses is generally believed to improve plate uniformity (spatially) and retard oxide morphology evolution in both cyclic and standby systems. There is, however, evidence which indicates that the use of short pulse alone, without any other reverse pulse patterns, can give rise to a plate surface morphology which differs from any stage in the normal "aging" cycle, enhances the formation of dendrites on the cell surfaces, and reduces the charging capacity of the cell.
Long (in duration) reverse current pulses between about 1 second to about 15 seconds, can also be incorporated in the charging system, as generally described before. The reverse pulses are long in duration compared to the initial step function response time of virtually all lead acid cells under most state-of-charge conditions. The long reverse pulses at moderate current, generally below 1C and typically below C/10, causes re-equilibrium within the cell (which could be provided by a prolonged zero current "rest" state at significantly higher efficiency) and reverses reactive gradients within the electrolyte. The efficient dissolution of dendrites plus the reversal of anomalies over physically large ranges, (many inches) helps maintain cell uniformity, and tends to reduce cell aging from a "worst case" rate to a "typical" rate. In addition, the use of long reverse pulses on long standby basis appears to reverse some cell inhomogeneities, such as sulfation "patches" caused by prolonged storage. Still another possible advantage of these long reverse pulses is the partial restoration of compromised cell capacity. As a practical matter, long reverse pulses must be used sparingly, both to conserve charge time and to limit the "make-up" demands on the charger. Interpulse periods in the order of minutes are generally appropriate.
While it is possible to store and use the energy withdrawn from a cell (or battery) by either short or long reverse pulses, the cost of energy reclamation is usually not justified by the monetary value of the reclaimed energy and the reduced reliability of the charger resulting from its increased complexity. Since the heat generated by this dissipation of energy is primarily external to the cell, it does not cause cell degradation.
"Rest" periods with no current flow can also improve battery characteristics. In cyclic battery service, there is generally little opportunity to introduce long rest periods since they require more rapid charging due to the typically limited time in which they are recharged. Even in standby battery applications, where long rest periods (in the range of minutes to hours) efficiently and economically reduce the diffusion gradient normal to plate surfaces, they do not reduce gradients parallel to plate surfaces. Therefore, stratification effects are generally not mitigated by zero current rest intervals. Also, even when using rest periods, it may be desirable to cause mechanical mixing by including very high current reverse pulses to induce local temperature anomalies and thermal diffusion or even brief very high current charging pulses to introduce gas bubbles throughout the system, especially in cells larger than about 100 Ampere-Hours (AH) per plate.
The range of effects which can be manipulated by the duration, current, and repetition rate of one or more reverse current pulse patterns, both short and long, are numerous. The previous examples illustrate a relatively simple way in which some of the choices can be economically implemented. Furthermore, serious loss of capacity mechanisms which become more noticeable toward the end of battery service life, such as plate sulfation, are effectively reduced by a multiple, reverse pulse environment. This can be an economical advantage since it delays battery replacement.
In expanding this discussion from cells to batteries (a small group of series connected cells--typically 3 or 6 cells in a lead acid type battery), effects resulting from cell differences, both initially and during operation, become significant. Cells can initially differ in both their capacity and their recharge efficiency. Additional cell differences arise due to differential aging of cells used in both cyclic and float service applications.
Continuous charging rates depend on small recombinant capacity of lead acid systems (especially sealed, starved electrolyte systems) which generally have a limit of about C/200. Therefore, a small continuous "trickle" current can effectively "complete" the charging of larger capacity cells in a series group. This is necessary because the "smallest" cells reach full charge first and raise the series string voltage to the charger cut-off limit before the larger capacity cells reach full charge. The trickle current, on the other hand, allows each cell to eventually reach full charge. Of course the trickle current must be larger than the self discharge current, which is typically about C/5000 to C/1000 depending on the cell construction and its operating temperature.
The difference between trickle current and self discharge current must be dealt with by recombination (or made up in refillable cells). By improving cell surface uniformity and reducing gradients, the self discharge currents and the required trickle current tend to be minimized, and the resultant recombinant capacity, i.e. the ability of the cell to receive current for ever, is minimized. Recombinant behavior is never perfect and leads to gas loss and consequent "drying".
Battery systems often contain long series of connected groups of batteries. All of the aforementioned problems with cells and single batteries are compounded in battery systems, especially since the differences between batteries tend to initially be larger and to grow more rapidly than the differences between cells in the same battery.
Long strings and/or systems with large cyclic requirements and "deep cycle" demands (discharging daily to as high as 85% of full charge in traction battery applications) may lead to catastrophic consequences as the initial battery differences grow with use. For example, a nominal 120 volt standby power system consisting of ten 12 V batteries, each with 6 cells in series, is subjected to discharge to 1.6 V per cell, i.e., a total discharge of 96 volts. The smallest cells (of the 60 in series) will be discharged well below 1.6 V and may even reach zero or be reversed before load cut-off occurs. This will reduce cell capacity slightly, but permanently because cells tend to be altered by deep discharge. Therefore, on recharge, the smallest cells will be somewhat overcharged before the total series assembly reaches high charge cut-off (at typically 2.30 V/cell) at 138 V. Some water loss will inevitably occur raising the specific gravity of the electrolyte and increasing local current activity and corrosion. With each cycle, the smallest cell will "shrink" more than the largest cells and the trickle current will become a larger fraction of the actual cell capacity and, therefore, closer to the recombinant limit. Eventually, the trickle current will approach the recombinant limit and rapid deterioration will follow because the extra charge current which is not recombined will cause effects such as corrosion, gas release, and decomposition of reactants.
This situation cannot be effectively improved, especially in cyclic service, by reducing the overall charging voltage since this will place the largest cells in an undercharged condition at the end of high charge. The trickle charge, which is only on for a limited period of time between battery usage, can generally not make up for a large deficiency in the charge capacity quickly enough to avoid undesirable changes in both plates. The introduction of periodic "equalization" charges, i.e., controlled overcharge, tends to aggravate the situation for the "smaller" cells while improving the condition of the larger cells.
The traditional "high performance" solution has been "group charging" where the series assembly is divided into subgroups, each provided with a separate charger. This may be as simple as one charger per 24 or 48 volt subgroup or as sophisticated as one charger per cell. While the results can be excellent, the cost is prohibitive for most applications, and the added complexity of the multiple systems can reduce reliability unless stringent quality control is imposed, further increasing cost.
In a traction battery system, a somewhat different form of the problem discussed with the standby system occurs. In this application, battery groups of typically 36 or 48 volts are 85% discharged in 16 hours and must be recharged in 8 hours daily. Discharge becomes damaging as the gradual reduction of cell capacity leads to discharge approaching 100% for the smallest cells. Rapid recharge requires sufficient current to assure full recharge of the largest cell within the allowed time. Therefore, the bulk of the cells (and possibly all of them late in life) are overcharged daily. This results in significant water loss which must be made up and generally precludes the use of sealed cells for this application. Flooded cells requiring continuous maintenance are the rule in this application, which has tolerated crude, brute force, chargers (which are economical and rugged but heavy, inefficient, and inaccurate) to flourish. Here again, while a charger per battery or cell would greatly improve performance, it would be at a cost and level of complexity, inappropriate to the field.
A serious need exists to provide a charging system which addresses the majority of the issues in lead acid charging without affecting the battery construction. Its design must provide the flexibility to economically apply those functions required for each specific application.